I am new to oracle. I am using oracle database 19c. I need to restrict the query execution in such a way that if a query is executed once, concurrent execution should get warning or wait until the first query is complete.
Example:
--First time execution
select count(*) from table;
--At the same time, another user execute the query
select count(*) from table; <<== Either wait or get a warning
Thanks in advance !
Related
A sql like "SELECT COUNT(1) FROM tablename..." was executed by my springboot application with mybatis,but no response during a long time.
But when I executed the same sql directly in ORACLE,it worked and returned the response.
With executing these sql:
select event,count(*) from v$session_wait group by event order by 2 desc;
I found the 'latch:cache buffers chains' event.
I asked my frined and he told me that I can execute analyze table tablename compute statistics.
After executed the sql,the problem resolved,the select count sql executed by springboot application worked !
I feel puzzled,why the analyze sql worked?
Thank you.
Parallel hints in normal DML SQL queries in oracle can be used in following fashion
select /*+ PARALLEL (A,2) */ * from table A ;
In similar fashion can we use parallel hints in PL/SQL for select into statements in oracle?
select /*+ PARALLEL(A,2) */ A.* BULK COLLECT INTO g_table_a from Table A ;
If i use the above syntax is there any way to verify whether the above select statement is executed in parallel?
Edit : Assuming g_table_a is a table data structure of ROWTYPE table
If the statement takes short elapsed time, you don't want to run it in parallel. Note, that e.g. query taking say 0.5 seconds in serial execution could take 2,5 second in parallel, as the most overhead is to set up the parallel execution.
So, if the query takes long time, you have enough time to check V$SESSION (use gv$sessionin RAC) and see all session with the user running the query.
select * from gv$session where username = 'your_user'
For serial execution you see only one session, for parallel execution you see one coordinator and additional session up to twice of the chosen parallel degree.
Alternative use the v$px_session which connects the parallel worker sessions with the query coordinator.
select SID, SERIAL#, DEGREE, REQ_DEGREE
from v$px_session
where qcsid = <SID of the session running teh parallel statement>;
Here you see also the required degree of parallelism and the real used DOP
YOu can easily check this from Explain Plan of the query. In case of Plsql you can also have trace of the procedure and check in the TKprof file.
I have an application that executes some sql queries. How can I get execution plan for currently executing query from sqlplus or some other oracle client? I can amend oracle session that is used in application, if that is necessary.
I do not want to use explain plan and execute that query by hand, I need to get actual execution plan that is used for query.
You can run explain plain on historical queries from the SGA -examples
And listing B.
Example:
SELECT username, prev_sql_id
FROM v$session
WHERE username = 'RDEVALL' -- example user
SQL ID returned
RDEVALL a1d4z5ruduzrd
RDEVALL avmbutuknvb6j
RDEVALL 75g0tqd9x743y
RDEVALL 5fnkh6c8mqqt3
RDEVALL 75g0tqd9x743y
Pick query ID and use here:
SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR('a1d4z5ruduzrd')); -- replace with sql ID as needed
I'm learning about database indexes right now, and I'm trying to understand the efficiency of using them.
I'd like to see whether a specific query uses an index.
I want to actually see the difference between executing the query using an index and without using the index (so I want to see the execution plan for my query).
I am using sql+.
How do I see the execution plan and where can I found in it the information telling me whether my index was used or not?
Try using this code to first explain and then see the plan:
Explain the plan:
explain plan
for
select * from table_name where ...;
See the plan:
select * from table(dbms_xplan.display);
Edit: Removed the brackets
The estimated SQL execution plan
The estimated execution plan is generated by the Optimizer without executing the SQL query. You can generate the estimated execution plan from any SQL client using EXPLAIN PLAN FOR or you can use Oracle SQL Developer for this task.
EXPLAIN PLAN FOR
When using Oracle, if you prepend the EXPLAIN PLAN FOR command to a given SQL query, the database will store the estimated execution plan in the associated PLAN_TABLE:
EXPLAIN PLAN FOR
SELECT p.id
FROM post p
WHERE EXISTS (
SELECT 1
FROM post_comment pc
WHERE
pc.post_id = p.id AND
pc.review = 'Bingo'
)
ORDER BY p.title
OFFSET 20 ROWS
FETCH NEXT 10 ROWS ONLY
To view the estimated execution plan, you need to use DBMS_XPLAN.DISPLAY, as illustrated in the following example:
SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY (FORMAT=>'ALL +OUTLINE'))
The ALL +OUTLINE formatting option allows you to get more details about the estimated execution plan than using the default formatting option.
Oracle SQL Developer
If you have installed SQL Developer, you can easily get the estimated execution plan for any SQL query without having to prepend the EXPLAIN PLAN FOR command:
##The actual SQL execution plan
The actual SQL execution plan is generated by the Optimizer when running the SQL query. So, unlike the estimated Execution Plan, you need to execute the SQL query in order to get its actual execution plan.
The actual plan should not differ significantly from the estimated one, as long as the table statistics have been properly collected by the underlying relational database.
GATHER_PLAN_STATISTICS query hint
To instruct Oracle to store the actual execution plan for a given SQL query, you can use the GATHER_PLAN_STATISTICS query hint:
SELECT /*+ GATHER_PLAN_STATISTICS */
p.id
FROM post p
WHERE EXISTS (
SELECT 1
FROM post_comment pc
WHERE
pc.post_id = p.id AND
pc.review = 'Bingo'
)
ORDER BY p.title
OFFSET 20 ROWS
FETCH NEXT 10 ROWS ONLY
To visualize the actual execution plan, you can use DBMS_XPLAN.DISPLAY_CURSOR:
SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST ALL +OUTLINE'))
Enable STATISTICS for all queries
If you want to get the execution plans for all queries generated within a given session, you can set the STATISTICS_LEVEL session configuration to ALL:
ALTER SESSION SET STATISTICS_LEVEL='ALL'
This will have the same effect as setting the GATHER_PLAN_STATISTICS query hint on every execution query. So, just like with the GATHER_PLAN_STATISTICS query hint, you can use DBMS_XPLAN.DISPLAY_CURSOR to view the actual execution plan.
You should reset the STATISTICS_LEVEL setting to the default mode once you are done collecting the execution plans you were interested in. This is very important, especially if you are using connection pooling, and database connections get reused.
ALTER SESSION SET STATISTICS_LEVEL='TYPICAL'
Take a look at Explain Plan. EXPLAIN works across many db types.
For sqlPlus specifically, see sqlplus's AUTO TRACE facility.
Try this:
http://www.dba-oracle.com/t_explain_plan.htm
The execution plan will mention the index whenever it is used. Just read through the execution plan.
I'm trying to analyse a query execution plan in my Oracle database. I have set
alter system set statistics_level = all;
Such that I can compare estimated cardinalities and times with actual cardinalities and times. Now, I'm running this statement in order to display that information.
select * from table(dbms_xplan.display_cursor(
sql_id => '6dt9vvx9gmd1x',
cursor_child_no => 2,
FORMAT => 'ALLSTATS LAST'));
But I keep getting this message
NOTE: cannot fetch plan for SQL_ID: 6dt9vvx9gmd1x, CHILD_NUMBER: 2
Please verify value of SQL_ID and CHILD_NUMBER;
It could also be that the plan is no longer in cursor cache (check
v$sql_plan)
The CHILD_NUMBER was correct when the query was being executed. Also, when I run dbms_xplan.display_cursor at the same time as the query, I get the actual plan. But my JDBC connection closes the PreparedStatement immediately after execution, so maybe that's why the execution plan disappears from v$sql_plan.
Am I getting something wrong, or how can I analyse estimated/actual values after execution?
You could always pin the cursor, which is new in 11g -
dbms_shared_pool.keep ('[address, hash_value from v$open_cursor]', 'C');
Increase the shared_pool to create more caching space for the cursors.
If in 11g, capture the sql plan in the baselines using optimizer_capture_sql_plan_baselines. This stores the plans in dba_sql_plan_baselines.